1. Technical Field
The invention relates to electrophoretic cells. In particular, the invention relates to mitigating electric field screening in the electrophoretic cells.
2. Description of Related Art
Electrophoresis is an electrokinetic phenomenon in which an electric field is employed to move a charged species. For example, particles dispersed and suspended in a fluid (e.g., a liquid or a gel) may have an associated net charge. Applying an electric field exerts an electrostatic Coulomb force on the dispersed particles that, in turn, induces a motion of the particles. A direction and speed of the induced motion of the particles is a function of a vector orientation of the applied electric field and a net charge associated with the particle. The function is often described in terms of an electrophoretic mobility.
Electrophoresis has a wide range of applications including, but not limited to, the separation of charged analytes in chemistry and biochemistry (e.g., gel electrophoresis, capillary electrophoresis, etc.) and the display of digital data. For example, electrophoretic displays, sometimes referred to as ‘e-paper’, employ the electrophoretic movement of pigment particles to display information. In general, electrophoretic displays may display information using electrophoretically implemented changes in either a reflection or a transmission characteristic of an electrophoretic cell (e.g., a pixel in the display). For example, a reflective electrophoretic display may display information by selectively moving charged pigment particles toward and away from a viewing surface (e.g., an electrode) of the display. Whether or not the particles are aggregated at the viewing surface effects an amount and, in some cases, a color of light that is reflected from the viewing surface. Transmissive electrophoretic displays often called ‘light valves’ modulate an illumination signal that passes through the electrophoretic cell by changing a density of suspended charged species that disrupt a passage of the illumination signal using electrophoresis. Chief among the advantages of electrophoretic displays is an inherently low power that is needed to maintain a particular display state.
Electrophoretic cells such as, but not limited to, those used in electrophoretic displays may suffer from electric field screening. Electric field screening occurs when a large number of charged species (e.g., charged particles) accumulate in a vicinity of one or more electrodes. The accumulated charged species screen the electric field and as such, reduce an amount of the electric field that penetrates the electrophoretic cell to further induce a desired motion of charged species toward or away form the electrode. A means for providing electrophoretic motion of particles in an electrophoretic cell that reduced or minimized screening would satisfy a long felt need.